Sometimes a guy will "accidentally" wear tiny shorts and no underwear and somehow manage to spread his legs wide in public. No control over what happens next.
Sack happens. Hello sack.
Or sometimes a guy will "not realize" his shorts, despite covering more leg, somehow have a hole right where the sack falls out.
Because he pulled his sack through the hole. Whoops! However could such a thing happen?! Gasp.
Then there's the bulge situation exacerbated by a partial erection. Then the erection being a public thing tends to make the erection go to the next level.
And so on.
As exciting as the idea of exposing oneself is, the act of it, the present tense of it, can be even more exciting.
Or make a guy scared as hell and totally stop. Sometimes fantasy is fantasy. Darn it.